Some wonders I don't build, because of how fast you/the AI can tech and obsolete the wonder. Like, Chinese American said, Stonehenge is a wonder that becomes obsolete fairly early, so its benefits don't really get used to their full potential. Some cases where I would build temples though, is maybe if your the Japanese, as you start with the ability to build temples, or if you discover the Ark of the Covenant somewhat early.
Other wonders are great, because their benefits last for awhile, like the Trade Fair. A few wonders don't get obsolete at all, so those ones are great, especially if you can rush them / get them early.
I don't however think that the cost of wonder's should get decreased. Even if it will become obsolete soon. You still get the bonus (albeit briefly), but you prevent anyone else from getting the wonder, and it counts towards cultural victory (which is already not that hard to achieve. If you made the cost of wonders even less, then cultural victory would be even more simple than domination).
How would you decrease the cost of wonders anyways? Because if you decreased the cost, shouldn't the benefit get decreased as well? Wouldn't that make them more like "better than average buildings"? I want a Wonder, something my civilization takes time and effort to build, and gives me a bonus or effect that no building can provide.